A lot of reviewers have focused on the fact that the core premise of the film (that humans only use 10% of their brain) is a myth, and as such the film is hard to believe - but we are living in a world where people are accepting radioactive spider bites, super soldier serums and genetic mutations as viable plot points. Films ask us to suspend our disbelieve all the time and Lucy is no different in this regard. Once the concept is established, Lucy sticks to it's own (admittedly, very loose) rules, and is used to create a fun, engaging sci-fi/action film that feels more than a little pulpy.
There isn't much to be said about the performances here - everyone is serviceable to the role they are playing. Scarlett Johansson plays a convincing, human version of Lucy before ingesting the drug, and plays post-drug Lucy as cold as you would expect, with just one scene, an emotional phone call to her mother, really standing out against the highly logical, almost Spock-like personality she takes on. Morgan Freeman lends gravitas the the film by just being Morgan Freeman - it seems so much easier to swallow sillier aspects of Lucy when Morgan Freeman is telling you that it makes sense.
That being said, the inspirations for the film are clear to see, with the first half of the film eerily reminiscent of Leon: The Professional and the finale going full 2001: A Space Odyssey. However, the similarities to 2001 remain surface level at best - there really is nothing to take away from Lucy, no deeper meaning hidden between the lines. But the film is no less fun for it, no less entertaining - the finale is tense, the action is stylish and Lucy herself has your full sympathy and support throughout the film. The plot moves at a real pace, and as it progresses the film takes an entirely unexpected turn, shooting in a direction you simply wouldn't expect it to, highlighting the films unpredictable nature.
Regardless of pseudo-science and surreal sequences, Lucy is oddly unique, entertaining throughout and a genuine surprise thanks to the direction the film takes. It's always nice to see another female-led action film, and hopefully the financial success of Lucy will prove that female-led blockbusters can work, opening doors for actresses and production companies (I'm looking at you, Marvel Studios). It's by no means Besson's best work, failing to come close to the excellence of Leon: The Professional, but Lucy is still an interesting, engaging film that isn't afraid to be different.